As is well known in aircraft technology, it is abundantly important to prevent gasses escaping from the aircraft engine from migrating into the nacelle and accumulating to the point where the gasses would combust and either cause fires or explosions. It is customary to purge the nacelle so as to avoid such occurrences. Current techniques for purging the nacelle is to locate a pump adjacent to the exhaust nozzle or at the front of the nacelle and purge the air that is contained in the nacelle through exhaust ports located at the tail end of the aircraft. Obviously, this entails pressurizing the stagnated gasses to assure that the proper exhaust flow is maintained throughout the operating envelope of the aircraft. The increase in pressure requires larger pumping apparatus resulting in heavier support structure that is necessary to attain the structural integrity of the aircraft and the engine parts which increases overall system weight and hence, incurs an engine operating performance deficit.
One type of pump heretofore utilized for this purpose is an ejector pump that utilizes fan discharge air as the primary fluid and dumps the entrained nacelle air directly overboard. The use of fan air for this purpose and in this manner penalizes engine performance as the fan air would otherwise be used for generating thrust.
Also well known in this technology is that there is concern in expending the cooling air after it has accomplished its cooling and ventilating functions. Ideally, it is desirable to dump the spent air into the gas path without incurring a penalty or more importantly to utilize any remaining energy in the spent air to augment the engine's thrust. It is also important in this technology to pump nacelle cooling air at flow rates and pressures sufficient to be entrained into the nozzle gaspath flow for sidewall cooling. Typically, one of two sources have heretofore been utilized for sidewall cooling. One of the sources is the fan air which is at a pressure higher than required to drive the flow and is also at relatively high temperatures which results in poor cooling and as mentioned above comes at the expense of engine performance. When fan air is used for cooling, the pressure must be throttled to minimize delta pressure (.DELTA.P) across the cooling liner. Thus, the energy that is used to raise the pressure of the fan air is essentially wasted when the pressure is reduced back to the required level. The other source is the ambient air that is entrained in the nacelle which has sufficiently low temperatures that would provide efficient cooling, but its pressure is too low for it to penetrate into the engine's gas path and flow therein.
We have found that we can obviate the problems enumerated in the above paragraphs and obtain sufficient sidewall cooling and nacelle purging without unduly sacrificing engine performance. The invention contemplates a supersonic jet pump using two ejectors in series that minimizes utilization of fan air and maximizes use of ambient air to attain sufficient flows at the required pressure level and including means for discretely dumping the cooling air in the gaspath.